The present invention relates generally to the field of high intensity discharge lamps, and more particularly to aspects related to overcoming vertical segregation in such lamps.
It has been shown previously that the vertical segregation present in long and thin cylindrical ceramic metal halide discharge lamps can be overcome by exciting the 2nd longitudinal acoustic mode of the lamp. In this regard, see U.S. Pat. No. 6,184,633. As an example, a 70 W ceramic HID lamp with dimensions of 4 mm ID and 19 mm IL was operated with a current frequency sweep from about 45 to 55 kHz with a 10 ms period. This 10 kHz frequency sweep is above the 1st azimuthal acoustic mode of the lamp and below the 1st radial acoustic mode of the lamp. In addition to operating the lamp in a stable manner this frequency range has the important benefit of straightening the arc when operated horizontally. The 2nd longitudinal acoustic mode was excited by amplitude modulating the 45 to 55 kHz frequency sweep at the power frequency corresponding to the 2nd longitudinal acoustic mode. The voltage (or current) waveform is cos(2xcfx80xcex94f1t)*[1+m3 cos(2xcfx80f3t)] where xcex94f1 is the 45 to 55 kHz frequency sweep, f3 is the amplitude modulating frequency (xcx9c24 kHz in this example) and m3 is the modulation index ( less than 1). The voltage (or current) spectrum is a frequency sweep from 45 to 55 kHz with two sidebands 10 kHz wide centered at +/xe2x88x92f3 (i.e. 26 and 74 kHz). This produces a power spectrum with frequency components at 2xcex94f1, 2xcex94f1+/xe2x88x92f3, and f3 It is the power frequency at f3 that excites the 2nd longitudinal acoustic mode. (Acoustic resonances are driven by a periodic power input. For sinusoidal type waveforms, the power frequency that excites an acoustic resonance is at twice the current frequency.)
The frequency of the 2nd longitudinal acoustic mode can vary by a few kHz from lamp to lamp because of small differences in mercury pressure, dimensions, chemistry, etc. The best frequency for color mixing can be found by probing frequencies around the expected frequency of the 2nd longitudinal acoustic mode and measuring the lamp voltage. At the frequency that gives good color mixing, the lamp voltage will be a maximum. An algorithm to find the 2nd longitudinal acoustic mode based on lamp voltage has been developed and is disclosed in U.S. Pat. No. 6,400,100, which disclosure is hereby incorporated by reference in its entirety.
A principal problem with configurations designed to excite the second longitudinal acoustic mode is the necessity for a relatively complex ballast.
Briefly, the present invention comprises, in one embodiment, a method to substantially overcome vertical segregation in HID lamps, comprising the steps of: determining and selecting a frequency sweep signal to produce arc straightening and a fixed frequency signal for exciting a second longitudinal acoustic mode; and exciting in conjunction with a carrier frequency an arc straightening acoustic mode together with a second longitudinal acoustic mode excitation.
In a further aspect of this embodiment, the frequency that produces arc straightening is in a range above a first azimuthal acoustic mode and below a first radial acoustic mode for the resonance spectrum of the HID lamp
In a further aspect of this embodiment, the step is provided of choosing the carrier frequency signal sufficiently high in frequency so that in conjunction with the frequency sweep signal the arc is stable and color mixing is achieved.
In a further aspect of this embodiment, the exciting step comprises amplitude modulating the carrier frequency signal with a sum of the frequency sweep signal and the fixed frequency signal.
In a further aspect of this embodiment, the step is provided of controlling an amount of arc straightening by controlling an amplitude of the amplitude modulating frequency sweep signal.
In a further aspect of this embodiment, the step is provided of controlling an amount of color mixing by controlling an amplitude of the fixed frequency signal.
In a further aspect of this embodiment, the exciting step comprises the step of summing the carrier frequency signal with the frequency sweep signal and the fixed frequency signal to obtain difference power frequencies which excite the arc straightening acoustic mode and the second longitudinal acoustic mode.
In a further aspect of this embodiment, the exciting step comprises the step of alternating in time continuously the carrier frequency signal, the frequency sweep signal and the fixed frequency signal, in any order, with both the frequency sweep signal and the fixed frequency signal being at one half the power frequencies required for arc straightening and color mixing, respectively.
In a further aspect of this embodiment, the step is provided of controlling an amount of arc straightening by controlling a duration of the frequency sweep signal relative to the duration of the carrier frequency signal.
In a further aspect of this embodiment, the step is provided of controlling an amount of color mixing by controlling a duration of the fixed frequency signal relative to the duration of the carrier frequency signal.
In a further aspect of this embodiment, the determining step comprises: determining a resonance spectrum for the HID lamp; selecting a frequency range for the frequency sweep signal to produce arc straightening that is above the first azimuthal acoustic mode for the HID lamp and below the first radial acoustic mode for the HID lamp and selecting a frequency for the fixed frequency signal to produce color mixing.
In a further aspect of this embodiment, the HID lamp has a cylindrical symmetry.
In a further aspect of this embodiment, the HID lamp has a discharge vessel with a ceramic wall.
In another embodiment of the present invention, an HID lamp with arc straightening is provided, comprising: a discharge vessel containing an ionizable filling; and a circuit for exciting in conjunction with a carrier frequency an arc straightening acoustic mode together with a second longitudinal acoustic mode in the discharge vessel.
In a further aspect of this embodiment, the exciting circuit comprises: a first component for generating a frequency sweep signal to produce arc straightening and a fixed frequency signal for exciting a second longitudinal acoustic mode and summing the frequency sweep signal and the fixed frequency signal; and a second component for combining the summed frequency sweep signal and the fixed frequency signal with a carrier frequency signal to excite the arc straightening acoustic mode together with the second longitudinal acoustic mode in the discharge vessel
In a further aspect of this embodiment, the second component provides a carrier frequency signal sufficiently high in frequency so that in conjunction with the frequency sweep signal the arc is stable and color mixing is achieved.
In a further aspect of this embodiment, the second component for combining amplitude modulates the carrier frequency signal with the sum of the frequency sweep signal and the fixed frequency signal.
In a further aspect of this embodiment, the second component for combining sums the carrier frequency signal with the frequency sweep signal and the fixed frequency signal to obtain difference power frequencies which excite the arc straightening acoustic mode and the second longitudinal acoustic mode.
In a further aspect of this embodiment, the second component for combining alternates in time continuously the carrier frequency signal, the frequency sweep signal and the fixed frequency signal, in any order, with both the frequency sweep signal and the fixed frequency signal being at one half the power frequencies required for arc straightening and color mixing, respectively.